1. Field of the Invention
The present invention relates to a printer for forming solder on electrodes of a semiconductor substrate by a printing method, and more particularly, to a micro-bump forming apparatus for printing using micro solder balls.
2. Description of the Related Art
In related art solder ball printers, as disclosed in Japanese Published Unexamined Patent Application No. 2004-186286, by a screen printing method, flux is applied to electrodes on a substrate surface, and solder balls are disposed thereon. The solder balls are then melted by reflow and fused on the electrodes.
Further, as disclosed in Japanese Published Unexamined Patent Application No. 2008-142775, in a structure in which solder balls are charged into mask openings by horizontally moving a squeegee head while rotating the same, a predetermined number of solder balls are fed to a rotating shaft of the squeegee head from a dispensing portion provided at an upper portion of the squeegee head, and the solder balls are fed from the rotating shaft onto a mask surface. In addition, Japanese Published Unexamined Patent Application No. 4-186791 discloses a structure in which a mask is made of a film, and solder in paste form is printed.
In recent years, the size of electrodes to be printed has become smaller, and also the size of solder balls used for printing has become smaller, in the range of 10 μm to 70 μm, relative to traditional solder balls having a size in the range of 80 μm to 150 μm. In response to this, it is necessary to reduce the size of the mask openings through which the solder balls are fed to a substrate. However, the known metal masks do not permit the formation of such small openings. Therefore, when solder balls are printed through large openings, the balls are likely to be moved off center with respect to the electrodes. As a result, there have been many cases of print failure being judged. In particular, in the case, as disclosed in Japanese Published Unexamined Patent Application No. 2004-186286, where flux is applied to electrodes, and then solder balls are mounted thereon, the adhesive force of the flux makes it impossible to correct the misaligned balls, resulting in the increase in print failure.
Further, in the method of feeding the solder balls onto the mask surface from the rotating shaft as disclosed in Japanese Published Unexamined Patent Application No. 2008-142775, the solder balls are dispersed on the mask surface along with the rotation of the squeegee head. Therefore, there is a problem that the solder balls cannot be always uniformly dispersed and arranged, causing print defects. Additionally, in Japanese Published Unexamined Patent Application No. 4-186791 having the structure in which the paste solder is printed using the film mask, the film is thin, having a thickness of 10 μm, and therefore cannot be applied to the solder balls having a diameter in the range of 10 μm to 70 μm.